One of the qualities of air within a space which makes that air more comfortable is its relative humidity (RH).
Experts generally agree that 70.degree. F. and 50% RH is perfect for most people, but somewhat lower temperature is acceptable particularly if the RH is near the ideal.
RH is the percentage of water vapor partial pressure in existing air to the saturation pressure of water vapor at the existing air temperature. As air temperature increases, RH decreases. Dew point temperature is the temperature at which RH becomes 100%, and is the temperature at which water vapor begins to condense from the air. There is a single dew point for every ratio of water vapor mass to air mass at a given pressure. These are well known principles, taught in every college thermodynamics and physics course series.
During hot summers in more humid areas the relative humidity tends to be too high, and air conditioning is commonly used to remove the excess humidity from the air. On the other hand, in colder climates during the heating season, maintaining adequate humidity is difficult. The low temperature of the outside air dramatically lowers the total amount of water vapor on a per unit mass of air basis, which the air can hold. Either intentionally or because of inherent leaks in the structure, the interior air is constantly replaced by this dry outside air. This causes the humidity level within the heated space to fall to uncomfortably low levels. For example, air at -20.degree. F. and 100% RH when heated to 70.degree. F. has RH of less than 10%, which is uncomfortably low. This is why people's skin tend to dry out so much when it's cold outside.
The response for many years to correct this situation has been to use a humidifier to add water vapor to the air. Cheapest are the small stand-alone humidifiers which are placed in a room to increase the air's humidity level. These require constant filling which is time-consuming and provides the "opportunity" for spilled water. Further, it is hard to accurately regulate the humidity level of a room served by a stand-alone humidifier. And a room humidifier provides humidity for little more than the room in which it's located. In a space having a number of rooms, room humidifiers are far too inconvenient for most people.
Another type of humidifier (in-duct) is installed in the heating system ductwork leading from a forced air type furnace. In-duct humidifiers are supplied with water from the structure's water supply, and hence require much less attention. A humidistat or humidity control can control the operation of the humidifier so that humidity level can be held to a preset value.
Particularly with in-duct humidifiers, the issue of too high humidity is important during heating seasons because of condensation on windows. Window surfaces are typically the coldest interior surfaces in the heated space, because they are the most poorly insulated. When the interior surface temperature of a window falls to a point below the dew point temperature in the space then condensation on the interior window surface occurs. If the difference between interior window surface temperature and dew point temperature is too high, so much condensation can occur that moisture runs onto the window frames, which can easily damage them over time. Therefore, it is necessary to limit the interior humidity level to a value whose dew point is below the window surface temperature. On the other hand, when the outside temperature is very low, the more closely enclosure dew point temperature can be held to just below window temperature, the more comfortable will be the interior space.
This is easier said than done for a number of reasons. In the first place, the changing outdoor temperature changes the window temperature and hence the maximum amount of water vapor acceptable in the interior's air. The changing outdoor temperature also changes the amount of water vapor in the outdoor air which replaces the indoor air. And temperature setbacks programmed into thermostats for energy-saving purposes can also dramatically affect the inside air temperature. While indoor air temperature does not affect its dew point, indoor air temperature does affect the window surface temperature. If for example indoor air temperature decreases, then the window surface temperature falls. If window temperature falls to below the dew point temperature of the air, then water vapor in the air will condense on the window surface.
It would of course be possible to directly measure the inside window surface temperature, the inside air temperature, and the inside RH, and calculate from the latter two the dew point. This approach can theoretically provide the most accuracy in controlling the inside RH to prevent window condensation. Unfortunately, there is no presently available device for conveniently measuring inside window surface temperature, and doing so may well be difficult.
A variation on direct window surface temperature measurement relies on an estimate of inside window surface temperature. It is known that the inside window temperature closely depends on the inside and outside air temperatures, so it is possible to estimate window surface temperature using these values. Inside air temperature is easy to measure of course. But outside air temperature is more difficult to measure for a number of reasons. Simply installing the outside sensor is expensive. Then too, if the outside sensor is not shielded from the sun and even the house, the measured value may not accurately reflect outside temperature. An outside sensor requires long leads, and these may be subject to noise, further degrading the accuracy.
Thus, the present state of the art does not provide inexpensive and accurate means for control of humidity within heated spaces so that indoor dew point temperature is consistently close to but still below the window temperature. As a result, in many installations, the indoor dew point may on occasion rise above the window temperature, causing condensation to form on windows. Other times, the humidity level may be held lower than is necessary, causing discomfort and even health problems for occupants.